This application generally relates to textured pattern sensing and detection, and more particularly, to using a charge-scavenging photodiode array for the same.
There exists a need in the fields of counterterrorism and law enforcement to identify and track suspected terrorists or felons from a distance, without the suspect's knowledge or cooperation, and without leaving a trail that might alert the suspect that he is under suspicion and by whom. A number of biometric sensing and tracking concepts have been proposed. For instance, remote fingerprinting has been identified as an attractive means for identifying and tracking of terrorists.
A problem, however, in remote, covert fingerprinting of a suspected terrorist or felon lies in the lack of contrast in detecting characteristics of the fingerprint. For instance, there may be insufficient differentiation in the reflectivity, emissivity, or polarization signature between the high points (or ridges) and the low points (or valleys) of the papillae to meet need under a broad range of conditions.
This is because passive sensors typically require some intensity, spectral, polarimetric or other form of image contrast to differentiate ridges from valleys in the dermal papillae. The subtle natural differentiation based on passive signatures may prove insufficient for discrimination except under very unusual conditions (e.g., shallow illumination grazing angles adequate to produce shadows). Many systems exist that create un-natural differentiation by selectively adding an artificial pigment to either the ridges or valleys. This is how traditional ink-on-paper fingerprints are taken.
Conventional 3-D Laser Detection and Ranging (LADAR) sensors, based on direct detection pulse-echo ranging techniques, may be considered for fingerprint detection. Ladar sensors typically include optical transceivers that use a laser to illuminate a target and an in-band optical receiver to measure the time-of-flight of the photons to and from the target to measure range. Three-dimensional (3-D) imaging ladars can operate in a “direct detection” mode, where a short pulse is transmitted to the target and the return pulse is incoherently detected by a photodetector such as an avalanche photodiode (APD) or an array of photodetectors. A 3-D image may be created by associating the measured range from a small region of the target with the position of that region in sensor angular coordinates. A single receiver channel can be scanned in two angular dimensions (such as in a television raster pattern) to produce the image. Alternatively, a one-dimensional photodiode array may be scanned in the orthogonal dimension to produce the image or a 2-D photodiode array may be used in a staring (non-scanning) fashion to generate the 3-D image. However, these direct detection ladar sensors typically do not have the range resolution necessary to measure the height difference between fingerprint ridges and valleys, which requires sub-millimeter accuracy. The counting capacity and switching speeds required for such a sensor are well beyond the capability of the most advanced high speed electronics.
Commercial ladar sensors are available which can record fingerprints using range contrast. These sensors determine whether the pixel is at the same range as a datum, such as a flat optical surface or at a different range. The size of a pixel (picture element) is the angle subtended by a resolution element in both angular dimensions. This “binary” range sensing approach can provide sharp fingerprint detail, but requires the suspect to place his/her finger or hand on a flat optical surface or window for scanning and therefore requires cooperation of the suspect and lacks covertness.
Alternatively, a ladar sensor may operate in a “coherent” mode where a coherent modulated optical waveform, such as an FM linear chirp, is transmitted to the target and the return waveform is mixed (or heterodyned) with a local oscillator beam at the detector to produce a beat carrier signal with the waveform modulation as sidebands. A processor may use a pulse compression circuit or algorithm to convert the transmitted and received chirps into electronic pulses and measure the time delay between these pulses to determine target range. To be an effective texture pattern sensing system for fingerprints and other low-relief targets, such a coherent ladar sensor would require a very high modulation bandwidth, well beyond the current state of the art and would also require the same extreme timing precision as the incoherent ladar. Furthermore, this approach is inherently very complex and would be both bulky and costly to implement.
Laser-based interferometric approaches to high resolution profilometry might also be used. However, these also require the use of a coherent source and heterodyne detection scheme with the same complexity, size, and cost disadvantages as other coherent ladar approaches.
These conventional approaches have been generally impractical in a terrorist-identification scenario where equipment cannot be pre-positioned, the range is quite variable, and/or the fingerprint must be taken remotely and covertly. The range variability may arise from several factors: the distance between the sensor and the suspect's fingers is not precisely known, the dermal papillae are on a quasi-cylindrical surface and therefore exist at different ranges, the finger is tilted with respect to the line-of-sight, and/or the target is in motion.